Some proposed high energy applications, such as high energy lasers and high speed long-range aircraft, have large cooling requirements with limited available electric or mechanical shaft power and limited available heat sinking for conventional vapour and air compression cycle cooling systems. High-energy laser systems have relatively low efficiencies that cause waste heat to be approximately ten or more times their beam energy. At the same time, they only operate effectively within stringent temperature ranges. High-speed long-range aircraft have large engine and airframe heat loads during the major portions of their flights, thereby consuming the normal capacity of any available fuel heat sink whilst their high speed makes any available ram air heat sink less suitable due to increasing temperatures and drag with increasing speed.
Some cooling systems have used the latent heat or enthalpy of vaporisation for a boiling liquid expendable to assist heat extraction. However, such systems have only been suitable for short-term heat loads, such as during supersonic dash flights.